In recent years, the luminance of a projection-type projector has been improved dramatically by adopting a high-efficiency high-pressure discharge lamp (hereinafter simply referred to as a lamp). However, the amount of a substance such as mercury, which is enclosed in the lamp tube, is increased for the purpose of higher efficiency of the lamp, and the impedance increases due to explosive gasification of the enclosed substance, so that the lamp is unstable immediately after the lamp is turned ON. In this situation, when the lamp is repeatedly turned ON and OFF, serious problems arise, such as a degradation in the lamp electrode, circuit destruction caused by an increase in noise due to repeated turning ON, and the like.
As a ballast for a discharge lamp, a low-frequency start type and a high-frequency start type are known.
The low-frequency start type ballast for a discharge lamp performs low-frequency driving to warm two lamp electrodes alternately for several seconds after the start of lamp operation. This type has the following drawback: due to the low frequency, there is a large difference in temperature between one lamp electrode being warmed and the other lamp not being warmed, so that current is likely to be interrupted when the polarity of current is switched. Therefore, the lamp is gradually transitioned to a stable state while the lamp is repeatedly turned ON and OFF, so that a considerably large load is put on the lamp electrodes.
On the other hand, the high-frequency start type ballast for a discharge lamp performs high frequency drive to warm two lamp electrodes equally for several seconds after the start of lamp operation. In this case, the temperature difference between the two electrodes is considerably small due to the high frequency, so that substantially no interruption occurs in lamp current.
Therefore, in the case of this start type, the load on the lamp electrode is considerably small.
FIGS. 10 and 11 are waveform diagrams illustrating temporal changes in a lamp current I (A), a lamp voltage V (V), and a lamp electrode temperature D (° C.) in the low-frequency start type and high-frequency start type ballast for discharge lamps, respectively.
In FIG. 10, the lamp current I (A) has a low-frequency start period of several Hz to several tens of Hz indicated by a period T1 immediately after the start of lamp operation, and this period is typically set to be 2 to 5 seconds. Thus, the conventional ballast for a discharge lamp commutation-drives the lamp current waveform I (A) with a low frequency of several Hz to several tens of Hz immediately after the start of lamp operation, and therefore, is called a low-frequency start type ballast for a discharge lamp. The conventional low-frequency start type ballast for a discharge lamp activates a lamp as follows. In FIG. 10, a breakdown current Id of the lamp current I (A) is increased by further raising a high-voltage pulse at the start of lamp operation as indicated by a period t3 of the lamp voltage V (V), thereby causing the lamp electrode temperature to rise rapidly as indicated by the lamp electrode temperature D (° C.) so as to stabilize the lamp at the start of lamp operation. However, due to the low-frequency start, there is a large difference in temperature between the two lamp electrodes, so that current is likely to be interrupted when the polarity of the current is switched.
As described above, the breakdown current Id through the lamp has a considerably and excessively large value during the unstable period t3 immediately after the start of lamp operation.
In addition, a sudden impedance change due to explosive gasification of a substance, such as mercury or the like, which is enclosed in the lamp, causes repetition of lamp current interruption and breakdown, leading to a degradation in the lamp electrode, circuit destruction due to the repeated breakdown current Id, and the like. On the other hand, the lamp efficiency conventionally is increased by techniques called “tapered electrode” and “short arc (reduced distance between the electrodes)”. Therefore, in the current situation, the degradation of the electrode during lamp operation due to the increase of the repeated breakdown current Id is not negligible.
By contrast, recently, a high-frequency start type ballast for a discharge lamp is becoming mainstream, which performs commutation drive with a high frequency (about several tens of kHz) with respect to the lamp current I (A) immediately after the start of lamp operation, and after a high-frequency start period T1, transitions to a period T2 of a steady commutation frequency (about 80 Hz to 400 Hz) appropriate for a lamp, with certain timing, as illustrated in FIG. 11. This high-frequency start period T1 is also typically 2 to 5 seconds as in the low-frequency start type ballast for a discharge lamp.
Concerning the circuit structure of the high-frequency start type ballast for a discharge lamp of FIG. 11, a choke coil having an inductance of about several tens of μH to several hundreds of μH is inserted in series to a lamp. When the lamp is broken down during the high-frequency start period T1, the choke coil provides a high-frequency impedance, thereby advantageously automatically reducing a breakdown current Ia in FIG. 11. However, at the same time, the choke coil serves as a low-pass filter during the high-frequency start period T1. Therefore, the waveform of the lamp current I (A) during the high-frequency start period T1 is a triangular wave (FIG. 11), which is different from the rectangular waveform (FIG. 10) of the lamp current I (A) during the low-frequency start period T1 (after the unstable period t3). As a result, even when a peak current value reaches a rated lamp current during the high-frequency start period T1, the effective value is about half. Therefore, the value of current for warming the lamp electrode immediately after the start of lamp operation is reduced to about half of that of FIG. 10. When an increase in the lamp electrode temperature is not sufficient during the high-frequency start period T1, it is highly likely that the lamp goes out at the timing t1 of switching to the steady commutation frequency period T2.
Further, in the high-frequency start type ballast for a discharge lamp, the relatively intermediate-size choke coil is driven by high-frequency switching during the high-frequency start period T1 of several tens of kHz, and therefore, a considerably large amount of magnetic flux energy is held in the choke coil. Therefore, with the timing t1 of switching from the high-frequency start period T1 to the steady commutation frequency period T2 of several tens of Hz to several hundreds of Hz appropriate for a lamp, a considerably large amount of excessively large current Ib flows due to a counter electromotive voltage occurring since the choke coil holds the magnetic flux, as illustrated in FIGS. 11 and 12, in part because a combined impedance of the lamp and the choke coil suddenly decreases. As a result, the lamp electrode is degraded. Here, FIG. 12 is a waveform diagram of the lamp current I (A) of the conventional high-frequency start type ballast for a discharge lamp, in which the lamp current I (A) is attenuated to zero after the timing t1 of switching from the high-frequency start period T1 to the commutation frequency period T2, so that the lamp goes out.
Although the above-described low-frequency start type ballast for a discharge lamp and high-frequency start type ballast for a discharge lamp each are used as a light source for a projector, it takes about one minute for the lamp illuminance to be increased to about 60% or more with which video can be viewed. Thus, it takes a considerably long time for video to be viewable, resulting in inconvenience to the user.
Here, FIG. 7 is a graph illustrating characteristics of an increase in illuminance versus a time elapsed from the start of lamp operation in the high-frequency start type ballast for a discharge lamp. Open triangles indicate a plot of the illuminance increase characteristics of a conventional example. It can be confirmed from FIG. 7 that it takes one minute or more for the lamp illuminance to be increased to about 60% or more in the conventional high-frequency start type ballast for a discharge lamp.
Therefore, the conventional high-frequency start type ballast for a discharge lamp has the following problems.
1. Concerning lamp operation for a projector, although they are video devices, it takes a considerably long time for the illuminance of the lamp to be increased, so that the user needs to wait for a considerably long time until the user can view video.
2. During the high-frequency start period, the effective lamp current value is half of that of the low-frequency start, so that the lamp electrode cannot be warmed sufficiently immediately after the start of lamp operation.
3. The lamp electrode is degraded due to an excessively large current occurring-when the control state is transitioned from the high frequency start period (high impedance period) to the steady frequency period (low impedance period).